MAGNETOSTRATIGRAPHY OF PERMIAN-TRIASSIC PARALIC SEDIMENTS FROM LANGZAI, GUIHZOU PROVINCE, SOUTH-WESTERN CHINA

We present results of a study of Permo-Triassic (P-T) magnetostratigraphy of paralic mudstone and carbonate sediments near Langdai, NW Guizhou province in the South China block. We have sampled at roughly 0.5 to 1 m-intervals for magneto-stratigraphy in two sections. The main section (which includes the P-T boundary) is located 300m above Zonghzai town, 12 km west of Langdai. For a fold test, a correlated section has been sampled 5 km south of Langdai (Tanghaishui section). More than 100 samples were stepwise thermally demagnetized. All samples display a present field direction at low temperature (NRM to 250°C) with less than 10% being heavily overprinted. Thermal stepwise demagnetization allowed us to isolate a magnetic component in 90% of the samples and objectively determine polarity in 80%. The composite section spans a reverse-normal (R-N) polarity sequence. Section-mean directions pass a fold test at the 95% confidence level, demonstrating that the magnetization pre-dates folding. The section-mean poles calculated for the Zonghzai section (l=48.3, f=216.9, A95=3.3, N=56) and the Tanghaishui section (l=38.6, f=209.8, A95=3.5, N=15) are very close to the mean P-T pole (l=48.8, f=227.7, A95=3.1, N=5) and mean-Lower Triassic pole (l=42.5, f=213.9, A95=2.7, N=10), respectively (Zhenyu and Besse, 2001). Petrographic investigations and rock magnetism indicate that both titano-magnetite and hematite carry this component. These results provide evidence that the remanence in these paralic sediments represents a primary magnetization. New magnetostratigraphic data in the marine Shangsi section (N Sichuan) indicate that a R-N polarity transition is constrained to within 50cm above the main extinction level. Based on this correlation, the R-N transition in the Zonghzai section is an excellent tool to constrain the biotic crisis in paralic P-T sediments. High-resolution isotopic dating on ash layers as well as biostratigraphical investigations in this section, in combination with ages and obtained on volcanic ashes in marine deposits in S China, are expected to further improve the correlation between these two sections, and also constrain the timing of the P-T biotic crisis in different paleo environments.